1) Field of the Invention
The present invention relates to a transfer apparatus that reads a scale, provided along the whole circumference of a belt that is made to rotate, by a sensor, and detects an actual speed of the belt based on information from the scale to correct a speed of the belt to a target speed according to the detected actual speed, and an image forming apparatus and a method of correcting the moving speed of the belt.
2) Description of the Related Art
Copying machines and printers as an image forming apparatus using an electrophotographic system have, in many cases, a function of forming a full color image according to increasing demands of the market.
The image forming apparatus capable of forming a color image includes a one drum type and a tandem type.
The one drum type of image forming apparatus includes a plurality of developing devices, which develop images with toners of colors, provided around one photosensitive element. The toners are deposited to latent images formed on the photosensitive element to form a full color composite toner image, and the toner image is transferred to a sheet as a recording material to obtain a color image.
The tandem type of image forming apparatus includes a plurality of photosensitive elements arranged in tandem and a plurality of developing devices that develop images with toners of different colors corresponding to the photosensitive elements. Single-color toner images are formed on the respective photosensitive elements, and the single-color toner images are successively transferred to a belt or a sheet to form a full color composite toner image.
The one drum type of image forming apparatus has one photosensitive element, and therefore, the whole of the image forming apparatus can be comparatively downsized, and the cost can be reduced accordingly. However, the one photosensitive element is made to rotate a plurality of times (four times for a full color image) to form a full color image on a sheet, which makes it difficult to increase the speed of image formation.
In the tandem type of image forming apparatus, the image forming apparatus requires a plurality of photosensitive elements, and therefore, the image forming apparatus tends to be upsized, and the cost is increased accordingly. However, the speed of the image formation can be increased.
As there is a desire to have image formation speed in the full color image formation as that in the monochrome-level image formation, much attention is now focused on the tandem type of image forming apparatus.
The tandem type of image forming apparatus employs a direct transfer system as shown in FIG. 22 or an indirect transfer system as shown in FIG. 24.
In the image forming apparatus of the direct transfer system, toner images formed on photosensitive elements 91Y, 91M, 91C, and 91K aligned in a row are sequentially transferred, by transfer devices 92, to a sheet of paper P carried on a sheet conveying belt 93 that rotates in the direction of arrow A, and a full color image is formed on the sheet P.
In the image forming apparatus of the indirect transfer system as shown in FIG. 24, toner images formed on the photosensitive elements 91Y, 91M, 91C, and 91K are sequentially transferred superposedly to an intermediate transfer belt 94 that rotates in the direction of arrow B, and the toner images on the intermediate transfer belt 94 are collectively transferred to the sheet of paper P, by a secondary transfer device 95.
When these two transfer systems are compared, it is obvious that the former has a disadvantage such that the whole configuration of the image forming apparatus is elongated in a direction of the sheet conveyance because a paper feed device 96 is provided on the upstream side of a plurality of photosensitive elements 91Y, 91M, 91C, and 91K and a fixing device 97 is provided on the downstream side thereof.
On the other hand, the latter has an advantage such that the image forming apparatus is downsized in its lateral direction (horizontal direction in FIG. 24), because as a secondary transfer position can be comparatively freely set, the secondary transfer device 95 and the paper feed device 96 can be provided under the intermediate transfer belt 94 as shown in FIG. 24.
Furthermore, in the former, if the image forming apparatus is tried to be made smaller in the lateral direction, the fixing device 97 has to be provided close to the sheet conveying belt 93. However, the front edge of the sheet P reaching a nip of the fixing device 97 is necessary to be warped so as to accommodate a difference in speed between the sheet conveying belt 93 and the fixing device 97 (the fixing device 97 moves slower). If the fixing device 97 is provided in the above manner, the distance from the sheet conveying belt 93 to the fixing device 97 is very short, and therefore, the shock, produced when the front edge of a thick sheet in particular reaches the fixing device 97, causes vibrations to occur over the sheet, and this easily affects an image.
On the other hand, in the latter, the secondary transfer device 95 can be provided under the intermediate transfer belt 94. Therefore, even if it is made smaller in the lateral direction, the image forming apparatus still has a space to dispose the fixing device 97 apart from the intermediate transfer belt 94. Consequently, even if the front edge of the sheet P reaches the nip of the fixing device 97, the sheet P can be warped to accommodate the difference, and therefore, the image is prevented from being badly affected thereby.
As explained above, the indirect transfer system of tandem type image forming apparatus is drawing attention because of its advantages.
In the tandem type of image forming apparatus, toner images of different colors formed on the photosensitive elements are superposed on the sheet or the intermediate transfer belt to form a color image. Therefore, if a position on which the images are superposed is deviated from a target position, color misalignment or a slight change in hue may occur in an image. Thus, image quality is degraded. Accordingly, the positional deviation (color misalignment) of the color toner images is a significant matter.
One of the causes of color misalignment is speed variations of the intermediate transfer belt in the case of the transfer apparatus of the indirect transfer system (sheet conveying belt in the case of the direct transfer system).
Japanese Patent Application Laid Open, (JP-A) No. H11-24507 (pages 3 to 4, FIG. 1) discloses a technology to correct speed variations of a transfer belt.
In this technology, a color copying machine is described such that an intermediate transfer belt (transfer belt) is rotatably supported among five support rollers including one drive roller, and toner images of four colors of cyan, magenta, yellow, and black are sequentially transferred superposedly to the circumferential surface of the transfer belt to form a full color image.
Provided on the internal surface of the transfer belt is a scale with scale marks finely and accurately formed thereon. The scale is read by an optical detector to accurately detect the moving speed of the transfer belt. The detected moving speed is feedback-controlled by a feedback control system so that the speed of the transfer belt becomes an accurately controlled moving speed.
However, even in the color copying machine described in JP-A No. H11-24507, toner fly-off inside the color copying machine may be deposited on the scale with time. Even if the scale has the finely and accurately formed scale marks, a sensor cannot detect such a toner-deposited scale, which causes the speed of the transfer belt to be deviated from a target speed. Thus, the color misalignment or the change in hue may occur in the color image.